1. Field of the Invention
The present invention relates in general to intake air control systems of engines, and more particularly to intake air control systems of a type that is applied to an internal combustion engine which has both an electronically controlled throttle valve whose angular position can be freely controlled and electromagnetically actuated intake and exhaust valves (viz., variable valves) whose open/close timing can be freely controlled.
2. Description of the Prior Art
In internal combustion engines, a throttle valve has been widely used for controlling the amount of intake air directed to cylinders (viz., combustion chambers) of the engine. In these days, however, for controlling the intake air amount, usage of electromagnetically actuated intake and exhaust valves has been proposed and put into practical use in place of or in addition to the throttle valve. That is, by controlling the open/close timing of the valves, particularly, the intake valves, the amount of intake air is controlled. One of systems that embodied such idea is disclosed in Japanese Patent First Provisional Publication 8-200025.
In the engines having such system, the throttle valve is not used, or even if used, the throttle valve is auxiliarilly used for the control of intake air amount. That is, for controlling the amount of intake air, operation of the intake valves is mainly used by controlling the open/close timing of the same. Under operation of the engine with this control, the interior of the intake passage is permitted to have a slight negative pressure, which lowers a pumping loss and thus improves a net thermal efficiency of the engine.
However, due to inevitable limitation of operation speed of the electromagnetically actuated intake valves, in a high speed cruising of an associated motor vehicle, it is difficult to control the intake air amount to a desired value by using only the operation of the intake valves. In view of this, a measure may be thought out wherein in accordance with the operation condition of the engine, the intake air control is switched to a mode wherein the intake air amount is mainly controlled by the throttle valve while reducing a controllable range of the open/close timing of the intake valves.
However, even in the above-mentioned measure, it inevitably occurs that in the certain operation condition of the engine inducing the main control by the throttle valve, throttling of the throttle valve inevitably induces a certain pumping loss of the engine, which brings about a larger lowering in engine torque than that induced by the operation of the intake valves. Thus, upon switching of the intake air control mode, undesired torque gap tends to occur which lowers the driveability of the engine.
It is therefore an object of the present invention to provide an intake air control system of an engine, which produces no torque gap or at least minimizes the same upon switching of the intake air control mode.
According to a first aspect of the present invention, there is provided an intake air control system of an engine, which comprises a throttle valve installed in an intake air passage; a variable valve incorporated with a cylinder of the engine, the variable valve being controllable to have a desired open/close timing; and a control unit which includes a mode providing section that provides a first control mode wherein an intake air control is carried out by controlling the open/close timing of the variable valve while keeping the throttle valve at a full-open or near full-open position and a second control mode wherein the intake air control is carried out by controlling the position of the throttle valve while reducing a controllable range of the open/close timing of the variable valve; an intake air amount calculation section that calculates respective target intake air amounts for the first and second control modes, the respective target intake air amounts thus calculated allowing the engine to output substantially same engine torque in the respective first and second control modes under the same operation condition of the engine; and a mode switching section that carries out a switching between the first and second control modes in accordance with an operation condition of the engine.
According to a second aspect of the present invention, there is provided an intake air control system of an engine, which comprises a throttle valve installed in an intake air passage; a variable valve incorporated with a cylinder of the engine, the variable valve being controllable to have a desired open/close timing; and a control unit that controls the throttle valve and the variable valve in accordance with an operation condition of the engine, the control unit being configured to provide both a first control mode wherein an intake air control is carried out by controlling the open/close timing of the variable valve while keeping the throttle valve at a full-open or near full-open position and a second control mode wherein the intake air control is carried out by controlling the position of the throttle valve while reducing a controllable range of the open/close timing of the variable valve, the first and second control modes being selectively switched in accordance with the operation condition of the engine, and the first and second control modes allowing the engine to output the same engine torque under the same operation condition of the engine, the first and second control modes being respectively provided by calculating respective target intake air amounts for the first and second control modes and feeding the engine with the calculated target intake air amounts respectively.
According to a third aspect of the present invention, there is provided a method for controlling an intake air amount for an internal combustion engine. The engine includes a throttle valve installed in an intake air passage and a variable valve incorporated with a cylinder of the engine and controllable to have a desired open/close timing. The method comprises providing a first control mode wherein an intake air control is carried out by controlling the open/close timing of the variable valve while keeping the throttle valve at a full-open or near full-open position and a second control mode wherein the intake air control is carried out by controlling the position of the throttle valve while reducing a controllable range of the open/close timing of the variable valve; calculating respective target intake air amounts for the first and second control modes, the respective target intake air amounts thus calculated allowing the engine to output substantially same engine torque in the respective first and second control modes under the same operation condition of the engine; and carrying out a switching between the first and second control modes in accordance with an operation condition of the engine.